Electrophotographic recording element having photoconductor with quenched luminescence during charging and method of making the photoconductor

ABSTRACT

AN ELECTROPHOTOGRAPHIC RECORDING ELEMENT HAS A PHOTOCONDUCTIVE LAYER THAT COMPRISES A NOVEL PHOTOCONDUCTOR WITH QUENCHED LUMINESCENCE DURING THE ELECTROSTATIC CHARGING THEREOF. THE PHOTOCONDUCTIVE LAYER MAY ALSO COMPRISE A LIGHT-SENSITIZING DYE THAT HAS A LIGHT-ABSORPTION BAND WITHIN THE RANGE OF THE LUMINESCENCE EMISSION OF THE PHOTOCONDUCTOR. A NOVEL METHOD OF MAKING THE PHOTOCONDUCTOR COMPRISES DOPING SUBSTANTIALLY PURE ZINC OXIDE WITH A FLUORIDE.

frame [Al/mam AWE/V577? Dec. 14, 1971 3,62 7,528

E. C. GIAIMO, JR. ET AL ELECTROPHOTOGRAIHIC RECORDING ELEMENT HAVING IHO'IOGONDUCTOR WITH QUENCHED LUMINESCENCE DURING CHARGING AND METHOD OF MAKING THE PHOTOCONDUCTOR Filed May 12, 1969 fan/4w (M/a5: 6/4/4/0, J2 6 07/140 [Ii/('6' mol- ATTORNEY 3,627,528 ELECTROPHOTOGRAIHIC RECORDING ELE- MENT HAVING PHOTOCONDUCTOR WITH QUENCHED LUMINESCENCE DURING CHARGING AND METHOD OF MAKING THE PHOTOCONDUCTOR Edward Charles Giaimo, Jr., and Simon Larach, Princeton, N.J., assignors to RCA Corporation Filed May 12, 1969, Ser. No. 823,906 Int. Cl. G03g 5/00, 7/00 US. Cl. 961.8 2 Claims ABSTRACT OF THE DISCLOSURE An electrophotographic recording element has a photoconductive layer that comprises a novel photoconductor with quenched luminescence during the electrostatic charging thereof. The photoconductive layer may also comprise a light-sensitizing dye that has a light-absorption band within the range of the luminescence emission of the photoconductor. A novel method of making the photoconductor comprises doping substantially pure zinc oxide with a fluoride.

BACKGROUND OF INVENTION This invention relates to an electrophotographic re. cording element having a photoconductor, and a method of making the photoconductor, and particularly, to recording elements of the zinc oxide and dye-sensitized zinc oxide types.

In copying images on electrophotographic recording elements, having a dye-sensitized photoconductive layer comprising zinc oxide, by an electrophotographic system employing corona charging means, it has been observed that small untoned (white) spots are sometimes present in the toned (black) areas of direct positive prints. In reversal-type prints, as for example those made on an electrophotographic recording element by cathode-ray recording, it has also been observed that the non-image (white) areas sometimes have toner deposited therein as toned (black) spots, thus degrading the quality of the reproduced images. While a few of these unwanted spots may be due to coating imperfections in the photoconductive layer on the electrophotographic recording element, it has been discovered that many of the spots are caused by the unwanted exposure of the dye-sensitized photoconductive layer due to a luminescence emission of the photoconductive zinc oxide during the electrostatic charging of the photoconductive layer.

Scintillating light spots have been observed occurring Within a photoconductive layer containing zinc oxide in the dark during its charging by a corona discharge. Depending upon the manufacture of the zinc oxide used, the light emitted can have different colors. Three colors have been observed; namely purple, blue, and green. The dimensions of the scintillating spots correspond to the unwanted (white) areas which are devoid of toner in the prior-art direct positive prints, and to the unwanted toned (black) areas in the non-image background of the priorart reversal prints.

SUMMARY OF INVENTION Briefly, in one embodiment of the novel electrophotographic recording element, a photoconductive layer comprises a novel photoconductor with a quenched luminescence characteristic during electrostatic charging thereof. In another embodiment, a photoconductive layer of the novel electrophotographic recording element comprises, in addition, a light-sensitizing dye that has a light-absorption band within the spectral range of the quenched luminescence of the photoconductor.

" nited States Patent 0 Patented Dec. 14, 1971 A novel method of making the photoconductor with a quenched luminescence characteristic during charging thereof comprises doping a substantially pure photoconductor, such as zinc oxide, with a dopant, such as fluoride. In one embodiment of the novel method, zinc oxide is doped by burning substantially pure zinc in the presence of oxygen and ffluorine. In another embodiment of the novel method, zinc oxide is doped by moistening it with a solution of a fluoride and heating the moistened zinc oxide.

The novel electrophotographic recording elements, having a dye-sensitized photoconductive layer comprising fluoride-doped zinc oxide, provide cleaner and sharper electrophotographic prints than prior-art recording elements because the luminescence emission that would otherwise occur during the charging is quenched. Thus, unwanted light is prevented from being absorbed by one or more of the light-sensitized dyes to expose and to spot the resulting prints. In addition to quenching, or markedly reducing, the luminescence of the photoconductor during charging, the fluoride dopant does not reduce the photoconductivity of the photoconductor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of apparatus used in one embodiment of the method of making the novel fluoride-doped Zinc oxide; and

FIG. 2 is a graph of the spectral distribution curves of zinc oxide, before and after doping with fluoride, and the reflectance vs. wavelength curve of a light-absorption dye to explain the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The novel electrophotographic recording elements utilize a novel photoconductor of fluoride-doped zinc oxide in their photoconductive layers. Hence, the novel recording elements have the advantages of providing a photoconductor with a quenched, that is, markedly reduced, luminescence during electrostatic charging. If the novel recording elements also have one or more lightsensitizing dyes in their photoconductive layers, the dyes may have light-absorption bands within the spectral range of the quenched luminescence during electrostatic charging of the novel photoconductor. The luminescence being quenched during charging, however, cannot degrade the quality of the images formed on the novel recording elements.

Referring now to FIG. 1 of the drawing, there is shown a schematic diagram of apparatus 10 for making the novel photoconductor of fluoride-doped zinc oxide by one embodiment of the novel method. Briefly, the novel photoconductor is made by burning substantially pure metallic zinc 12 in an atmosphere of oxygen and fluorine gases. A quantity of substantially pure zinc 12 is placed in a quartz vessel 14. The upper portion of the vessel 14 communicates with the lower portion of a cone-shaped quartz reaction chamber 16 through a conduit 18, and the upper portion of the reaction chamber 16 communicates with a conduit 20 for the purpose hereinafter appearing.

The vessel 14 is swept free of air by a stream of inert gas, such as argon (A) or nitrogen (N for example, injected into the vessel 14 through a conduit 22. Any air within the vessel 14 is swept through the conduit 18, the reaction chamber 16, and out through the conduit 20.

The zinc 12 is heated to a temperature of between 910 C. and 1000 C. by any suitable means, as represented by the wavy arrows 24. The zinc 12 vaporizes at this temperature, and zinc vapor passes through the conduit 18 and into the reaction chamber 16. The reaction chamber 16 is also heated, preferably by a separate temperaturecontrolled furnace (not shown), to a temperature of between 450 C. and 1000 C., as represented by the wavy arrows 26. A temperature of about 600 C. in the reaction chamber 16 is preferred.

In the reaction chamber 16, the zinc vapor reacts with oxygen and fluorine to form the novel photoconductor, fluoride-doped zinc oxide. The oxygen and fluorine gases are introduced into the reaction chamber 16 through a mixing vessel 28. Air may be substituted for the oxygen. The oxygen and fluorine gases are introduced into the mixing vessel 28 through separate conduits 30 and 32, respectively. The mixed oxygen and fluorine gases are then introduced into the reaction chamber 16 through a plurality of conduits 34 and 36, for example, which communicate with the outlet of the mixing vessel 28 and the reaction chamber 16. The quantity of fluoride dopant used is not critical.

While fluorine gas is preferred as a dopant, fluorides, such as hydrogen fluoride, ammonium fluoride, and zinc fluoride may be decomposed by thermal decomposition to yield the volatile fluoride dopant. The amount of oxygen introduced into the reaction chamber 16 is that calculated on a stoichiometric basis to produce zinc oxide within the reaction chamber 16. The ratio of fluorine to oxygen used is such as to provide fluoride-doped zinc oxide having between percent and one percent of fluoride by weight to the weight of zinc oxide formed. This amount of fluoride dopant in the zinc oxide is adequate to quench the luminescence during electrostatic charging Without substantially affecting the photoconductivity of the zinc oxide. The zinc oxide doped with fluoride is swept through the reaction chamber 16, by the heat and gases therein, into the conduit to a container (not shown). A suction device (not shown) may be attached to the conduit 20 for extracting the fluoride-doped zinc oxide formed within the reaction chamber 16.

In another embodiment of the novel method, fluoridedoped zinc oxide is produced by moistening substantially pure zinc oxide with a dilute solution of a soluble fluoride, such as, for example, potassium fluoride, or ammonium fluoride, and subsequently heating the moistened zinc oxide. For the doping to be elfective, the moistened Zinc oxide should be heated to a temperature sufficiently high to allow the migration of the ions of the doping agent, fluoride, into the zinc oxide lattice. The temperature at which ionic migration into zinc oxide begins is about 450 C. Therefore, the moistened Zinc oxide should be heated to a temperature of at least 450 C. and maintained at this temperature for a period of about two hours.

Referring now to FIG. 2 of the drawings, there is shown an unbroken curve 40, representing the cathodoluminescence emission spectrum of a sample of zinc oxide made by the French process and widely used as a photoconductor in electrophotographic recording elements. A corona charging of this sample was simulated by an electron beam operated at 9 kv. and 1 a, and the intensity of the cathodoluminescence emission spectrum was measured in arbitrary units over a range of wavelengths from 3600 A. to 5800 A. The spectral distribution of the curve 40 consists of two major bands, one (3600 A.-4200 A.) in the UV band with a peak wavelength at about 3900 A., and the other (4400 A.-6000 A.) in the visible (green) band with a peak Wavelength at about 5200 A. These bands are designated herein by their peak wavelengths.

The 5200 A. band from zinc oxide is believed to arise from oxygen vacancies, or interstitial 2m, in the zinc oxide lattice. This band was eifectively eliminated by difiusing fluoride ion into the zinc oxide, as by the aforementioned methods of doping zinc oxide with fluoride. The intensity of the cathodoluminescence emission spectrum of the fluoride-doped zinc oxide is quenched, and is shown by the broken-line curve 4, indicating substantially zero luminescence emission in the visible spectrum (5200 A. band).

A reflectance curve 42 (the reciprocal of its lightabsorption curve) of a light-sensitizing dye, fluorescein, is also shown in FIG. 2. The intensity (in arbitrary units) of the reflectance curve 42 is plotted over a range of wavelengths from 4000 A. to 5800 A. From an inspection of the curves 40 and 42, it is apparent that the fluorescein dye on a photoconductive layer of zinc oxide would absorb any luminescence emission induced by electrostatic charging of the undoped zinc oxide in the 5200 A. band. Such light absorption would expose the photoconductive layer to (non-image) unwanted scintillation and produce unwanted spotting in the developed print.

Eliminating the 5200 A. band from the zinc oxide by doping it with fluoride provides a novel photoconductor for improved recording elements. Samples of electrophotographic recording elements were prepared with zinc oxide having the 5200 A. band luminescence emission induced by electrostatic charging and without the 5200 A. band luminescence emission. These samples were electrostatically charged, exposed and toned. Magnified photornicrographs (positive prints) of the toned surfaces of these samples were examined, and those samples that contained the photoconductive layers of zinc oxide with the unquenched 5200 A. band luminescence emission during electrostatic charging and at least one light-absorbing dye showed an increase in the size and number of the spots of insufficient charged density to allow toner to be deposited in comparison to those samples that did not contain the 5200 A. band. Also, when the charged and exposed samples were toned with a reversal type toner, the samples of the recording elements that contained the zinc oxide without the 5200 A. band luminescence during electrostatic charging had fewer spotted toner deposits in the white (non-image) background areas than the recording elements that contained the zinc oxide with the 5200 A. band luminescence.

The emission band at 3900 A. in the ultraviolet spectrum has been observed previously in cathode-ray excited photoconductors, and is believed to be due to the ion bombardment of the zinc oxide from the corona discharge. From the aforementioned results, it would appear that a mismatch between the luminescence emission during electrostatic charging and the dye absorption bands is desirable for electrophotographic recording elements of improved quality. Eliminating the 5200 A. emission hand during electrostatic charging in the zinc oxide of the photoconductive layer is thus desirable, especially when one or more visible-light-absorbing dyes, such as fluorescein, erythrosin, and rose bengal, for example, are used for sensitizing the electrophotographic recording elements.

We claim:

1. In an electrophotographic recording element of the type adapted to receive an electrostatic charge on the surface of a photoconductive layer comprising zinc oxide which in the absence of a suitable dopant has a luminescence emission characteristic during charging, the improvement comprising:

all of said photoconductor being doped with fluoride ions diffused within the lattice of said zinc oxide in an amount between about l0 weight percent and 1.0 weight percent of said zinc oxide to quench the luminescence emission thereof during the charging.

2. An electrophotographic recording element having a photoconductive layer comprising a photoconductor with quenched luminescence during the electrostatic charging thereof, and a light-sensitizing dye having a light-absorption band within the spectral range of said quenched luminescence of said photoconductor,

all of said photoconductor of said layer with quenched luminescence comprising zinc oxide doped with fluoride ions diffused throughout the lattice of said zinc oxide, and

the amount of fluoride ions in said zinc oxide lattice comprising between about 10- percent and 1 per- 6 cent, by weight, with respect to the weight of said FOREIGN PATENTS Zmc OXlde- 266,466 1/1964 Australia 96-1.5 References Cfied 684,487 4/1964 Canada 252 501 UNITED STATES PATENTS 5 2,879,182 3/1959 Pakswer et a1. 252 501 X CHARLES VAN HORN Prmary Exammer 3,041,166 6/1962 Bardeen 252501 X I. R. MILLER, Assistant Examiner 3,197,307 7/1965 Blake et a1. 961.8 X 3,232,755 2/1966 Hoegl et a1. 252-501 X 3,312,548 4/1967 Stranghan 96-1.5 10 23 147; 9 1 7; 252 501 

